The present invention relates to a multilayer ceramic substrate suitable for use in the production of electronic components, particularly those having a hybrid integrated circuit, and a process for the manufacture of such a substrate.
A multilayer ceramic substrate for use to form a hybrid integrated circuit comprises at least one ceramic layer as an insulator and at least one internal conductor layer of a first conductor. Each ceramic layer has at least one opening called through hole or via hole which is filled with the first conductor in order to obtain electric continuity between adjacent conductor layers. The exposed surface of the first conductor filled in the opening in the surface ceramic layer is covered with a thick film layer of a second external conductor.
Multilayer ceramic substrates are mainly manufactured by the sheet laminating method or print laminating method. In the sheet laminating method, a multilayer ceramic substrate is manufactured by preparing a plurality of green ceramic sheets each having a printed pattern of a first conductor thereon and at least one through hole filled with the first conductor, stacking and laminating these sheets, sintering the resulting multilayer structure, and applying a second conductor so as to cover the exposed surface of the first conductor. In the print laminating method, the multilayer structure is formed by printing one or more first conductor layers and one or more ceramic insulator layers alternately on a green ceramic sheet.
Since the internal first conductor layers are heated at a high temperature together with the green ceramic sheets when the multilayer structure is sintered, the first conductor is based on a refractory metal such as tungsten or molybdenum having a high melting point so as to avoid melting of the conductor during sintering. The second conductor can be based on silver or other noble metals.
These conventional multilayer ceramic substrates have the disadvantage that they have a high contact resistance and a low bonding strength between the first and the second conductor layers, particularly when the exposed surface of the first conductor layer is oxidized.
In order to overcome this problem, it has been proposed that a Cu-based material is used to form the second conductor since it can be fired in a nitrogen atmosphere so as to prevent oxidation.
Furthermore, in order to improve the reliability of mechanical and electrical connection between the first and second conductors, it has been attempted to interpose a metallic layer between these two conductors. The metallic layer is formed on the exposed surface of the first conductor layer of the multilayer structure, i.e., in the vicinity of through hole areas of the surface insulator layer.
For example, Japanese Utility Model Application Kokai No. 57-12775(1982) discloses that a metallic layer which may be comprised of a single nickel layer or a combination of a nickel layer and a gold layer is interposed between the first and second conductors. The metallic layer is formed by electrolytic or electroless plating or vacuum evaporation.
Japanese Patent Publication No. 63-42879(1988) discloses that a metallic layer of nickel, cobalt, or copper having a thickness of from 0.2 to 5 .mu.m is formed by electrolytic or electroless plating on the exposed surface of the first conductor layer. Similarly, Japanese Patent Application Kokai No. 51-133766(1976) discloses a metallic layer formed by plating with a metal such as nickel, copper, or silver and having good wettability with the first conductor layer.
The metallic layer described in Japanese Patent Application Kokai No. 59-171195(1984) has a nickel plating layer and a silver- or gold-containing noble metal layer formed thereon. The noble metal layer can be formed by placing a noble metal foil, or plating, or printing with a thick film noble metal paste. Similarly, Japanese Patent Publication No. 1-48672(1989) discloses the formation of a metallic layer by nickel plating followed by printing with a thick film nickel paste.
These methods for the formation of a metallic layer between the first and second conductors involve various problems.
Electroless plating is accompanied by undesirable extension of the plated layer by bleeding from the through hole area to a surrounding area. In addition, the plated metal tends to diffuse toward the surface of the second conductor layer, thereby deteriorating the wettability of the surface by a solder which is required at a later stage.
Electrolytic plating causes mechanical and electrical problems such as a restriction in the dimensions of the substrate or an increase in the stray capacity because lead wires must be connected to perform the plating. Furthermore, such wiring makes the manufacturing process more complicated and the above-mentioned deterioration in wettability by a solder is also inevitable.
In the plating method, a plating solution is used irrespective of the type (electroless or electrolytic). When the exposed surface of the first conductor layer that is the surface to be plated has depressions as indicated in FIG. 5 by the numeral 8, the plating solution may remain in the depressions 8. In this case, the quality of the resulting multilayer ceramic substrate may be degraded due to discoloration of the outer surface of the substrate or oxidation of the conductor layers.
Vacuum evaporation is not an efficient method and deteriorates the productivity.